The oldest and best known type of rechargeable battery is the lead-acid battery. While the present invention is not so limited, it has been developed as an improved lead-acid type battery. Accordingly, the description is primarily in terms of such a battery.
A typical lead-acid battery comprises a positive electrode, a negative electrode, one or more separators, and an electrolyte. The electrodes function both as electrical contacts and as mechanical load-bearing elements. Each electrode is formed by coating a lead or lead alloy grid with an active paste material. The paste dries to form a porous layer of the active material as part of each electrode.
A separator may be any porous, perforated, or fibrous material that sufficiently isolates the electrodes to prevent short circuiting. However, the separator must also be sufficiently open to permit ion transfer through the electrolyte contained in the separator. Perforated plastic, or glass fiber, sheets are commonly used as separators. A compressed mat of glass fibers is currently used in many commercial storage batteries.
Porous earthenware and sintered silicate sheets have also been proposed. However, they have not been adopted commercially to any significant extent. One problem has been lack of sufficient porosity to permit proper operation of a battery.
The electrolyte may be any ionic medium that can provide ion transfer between the electrodes. In a lead-acid battery, sulfuric acid is the electrolyte employed.
A battery may be packaged in a plastic case for insulating purposes. However, the electrodes constitute the primary mechanical support and load-bearing means in current storage battery construction.
The glass fiber mat, now in use as a separator, has certain desirable features. It readily takes up and holds electrolyte, a property commonly referred to as wettability or wickability. It is also resistant to attack by the electrolyte, and provides acceptable electrical properties.
The fiber mat separator is, however, flexible and lacking in mechanical strength. This means that the electrodes, the casing, or other support members must be the primary source of structural integrity in a battery.
Batteries are commonly classified as either a flooded type or a starved, or sealed, type. In both types, the electrodes are in contact with the separator and held in that assembly. The porous, active material coating on the metal grids, as well as the separator, become saturated with electrolyte. In the flooded type, the electrode and separator assembly is immersed in excess electrolyte so that the open space around the assembly is filled with electrolyte, e.g. sulfuric acid. In the starved, or sealed, type, the electrolyte is completely contained within the pores of the separator and electrode paste. In this construction, it is important that the electrolyte be retained in the pores to avoid leakage of the corrosive acid electrolyte.
Our second related application Ser. No. 08/506,713 describes a rechargeable battery assembly comprising an elongated, rigid, porous, ceramic separator. The separator has a honeycomb structure in which open cells are separated from adjacent cells by thin, porous, ceramic walls, the open cells and separating walls running lengthwise of the honeycomb separator. The cell walls are porous, and the open cells and wall pores are available to be filled with an electrolyte to permit ion flow between electrodes in a battery. In this assembly, electrodes are applied externally, that is, to the side walls of the separator.
The present invention is also based on a battery assembly employing a porous, ceramic, honeycomb body as a separator. In the present battery, however, electrodes are assembled internally, that is, within the cells of the honeycomb body.